With the advent of mobile computing devices, and the rapid growth in applications involving group communication (electronic classrooms, MobiTV, quad-play cable services, social networking, among others), efficient link layer multicasting is emerging as a key requirement for wireless networks. As opposed to existing broadcast/multicast viewership models, users are increasingly demanding the ability to view media content on wireless devices, at a time and/or place of wiring costs in existing environments targeting group communications such as conference rooms, lecture halls, for example.
While the broadcast nature of the wireless medium intrinsically supports multicasting, link performance is dictated by that of the weakest client(s). With their ability to maximize performance in low signal-to-noise ratio (SNR) environments, beamforming antennas provide a natural solution to this problem. Beamforming antennas focus the signal energy in specific directions to provide an improved SNR (array gain) at the client. Beamforming could be either adaptive where the beam patterns are computed on the fly based on channel feedback, or switched, where precomputed beams are used. Switched beamforming, which provides a balance between performance and complexity, aims to radiate energy in a desired direction towards the receiver. While the performance gains from switched beamforming are expected in line-of-sight (LOS) rich environments outdoors, gains can be had even within an indoor environment. In fact, due to the strong LOS nature of indoor channels, switched beamforming has been made a defacto choice in some of next generation indoor wireless standards.
There is an inherent tradeoff between multicasting and beamforming. While beamforming provides increased signal strength (array gain), leading to increased data rates, it reduces the signal footprint, thereby restricting the wireless broadcast advantage (WBA)—a key factor for multicast applications. On the other hand, omni-directional transmissions, while completely leveraging the benefits of WBA, are not able to avail any of the array gain and the consequent data rate increase. This makes the problem of multicasting with beamforming antennas especially challenging.
FIGS. 1(a)-(c) show two exemplary topologies with an eight element antenna at the access point (AP). In the topology 1 of FIG. 1(a), the RSSI at clients A and B due to the exemplary eight element AP is 8 dB and 10 dB respectively, which corresponds to an omni RSSI of 2 dB and 4 dB respectively (with a 6 dB array gain) in this example. For 802.11b rates, a packet of L Mbits would require a total time of L (seconds) at a bottleneck rate of 1 Mbps for the omni strategy (SO). On the other hand, with SB, a transmission on beam 1 to client A at 2 Mbps, followed by a transmission on beam 2 to client B at 5.5 Mbps yields a total time of only 0:68 L, which is a 32% gain over omni.
On the other hand, FIG. 1(a) using topology 2 with four clients (A, B, C, D) having RSSI of 8.5, 12, 8.5, and 12 dB respectively. While SO would require a total time of L, SB would incur four transmissions; two at 2 Mbps rate each and the other two at 11 Mbps rate each, resulting in a total time of 1:182 L, which is a 20% loss compared to omni that can be attributed to the loss in WBA.
Thus, while beamforming has the potential to improve multicast performance, failure to address the tradeoff with WBA could even degrade its performance worse than omni.